Suspended and compact fiber optic sensors

ABSTRACT

A compact, optically double-ended sensor probe with at least one 180° bend provided in the optical fiber in close proximity to a fiber Bragg grating temperature sensor suspends the optical fiber within a casing in such a way that the expansion and contract of the probe casing will not materially influence the temperature reading of the fiber Bragg grating by adding time varying or temperature varying stress components.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.12/555,006 filed Sep. 8, 2009 entitled “Compact Fiber Optic Sensors AndMethod Of Making Same,” which application claims the benefit of priorityfrom provisional application No. 61/095,885 filed Sep. 10, 2008, thecontents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

FIELD

The technology herein relates to fiber Bragg grating optical temperaturesensors fabricated in the cores of high-silica optical fibers, and totechniques for making such sensors compact enough in form factor tocompete economically with, and be used physically in place of, commonsmall electronic sensors in various applications while preserving theiradvantages in multiplexing on a single fiber.

BACKGROUND AND SUMMARY

Many or most single mode communications-grade optical fibers and manymulti-mode fibers are fabricated from high-silica glass components. Suchfibers have a high Young's Modulus, and are termed nearly “perfectlyelastic” in addition to possessing very low thermal coefficients ofexpansion. This combination of properties makes the optical fiber quitestable for communications purposes in the field if precautions are takento protect it from moisture-caused static fatigue failure, hydrogendiffusion (causing higher absorption of light) and physical forces,among other dangers. Such protection means include, but are not limitedto, coating (e.g., during the fiber drawing process) with materials suchas acrylates, polyimides, carbon, diamond-like carbon, copper, aluminumand other materials that can be applied to the fiber during the highspeed drawing process. These coatings are usually termed “buffer”coatings. Subsequently, the fibers are frequently cabled or jacketedwith materials that include strength members (e.g., Kevlar fibers) andjackets for crush and kink protection.

Such fibers often include in their structures at least one core with atleast one index of refraction and at least one glass cladding adjacentto the core with at least one index of refraction that is lower thanthat of the core in order to substantially confine light to the core.

Optical fiber sensors of temperature and/or strain based on common fiberBragg gratings (“FBGs”) can be fabricated in the cores of optical fibersby various means. These gratings are characterized by alternatingregions of index of refraction value along a longitudinal length of thefiber core having some pitch, or period. There are several distincttypes or varieties of FBGs, including but not limited to short period,long period, blazed and phase shifted gratings. Further, these types canbe modified by varying the period (chirp), amplitude (apodizing), indexbackground level and/or physical damage level used to fabricate thegratings. Such damage can be induced by a higher intensity of the FBGfabricating light (usually ultraviolet, or UV lasers; in some cases CO₂or other sources) than is actually necessary to write the grating. Thenumber of cores, core shapes, number of cladding layers, and addition ofstress-inducing members can all be varied to control the opticalproperties for various applications. Different elements can be added tothe glass formulation to control the index contrast between the core(s)and the cladding(s).

Advantages of optical sensors over electronic sensors are generally wellknown, in spite of their present overall greater cost (including thesensor readers). Such advantages include, but are not limited to,immunity to electromagnetic interference (EMI) and electromagneticpulses (EMP), corrosion resistance, explosion-proof nature, lightweight, small size and potential for all-dielectric construction(leading to high voltage compatibility). In addition, sensors based onFBGs enjoy the ability to be multiplexed on a single optical fiber inlarge numbers by several means, including wavelength divisionmultiplexing (WDM) and optical frequency domain reflectometry (OFDR),leading to a lower cost per sensing point when the cost of the readinginstrument is averaged over the number of sensors attached. Further,only a single feedthrough point through bulkheads and pipes is neededfor a high sensor count, leading to enhanced ease of installation andlower vulnerability to breach of the bulkhead integrity at thefeedthrough. In order to be multiplexed in this way, physically inseries along the fiber, the sensors should generally be optically doubleended, or have an input fiber and an output fiber (it is understood thatthe input and output fibers are interchangeable for an FBG). In order tomake FBG sensors both small enough to be compatible in form factor withelectronic sensors and optically double ended requires innovation beyondthe present state of the art.

Most types of FBGs are sensitive to both temperature and strainvariables to essentially the same degree for a given type, although thedegree of interdependence on the two variables may vary from type totype. Further, if the FBG is fabricated in the core of a high-silicafiber, such as is commonly done, the sensor also has the properties ofhigh Young's Modulus and low coefficient of thermal expansion. Theseproperties generally cause difficulty if the sensor is to be used over avery wide temperature range, if their temperature sensitivities ortemperature ranges need to be enhanced beyond that of the simplebuffered fiber (by attachment to a material of a higher expansioncoefficient), if they will be subjected to rough handling, or firmlymounted to dissimilar materials (to enhance thermal equilibrium with theobject to be measured). In addition, fabrication difficulties increasewhen the effects of strain are to be separated unambiguously from thoseof temperature and when the sensor is made compact enough to competewith existing electronic sensors in form factor while still maintainingtheir ability to be multiplexed.

If a section of optical fiber containing an FBG is attached to anotherobject or material (substrate) with adhesive or even thermal grease, theFBG's temperature calibration and even repeatability is significantlyand usually adversely affected by all the components of the attachmentsystem, especially over a temperature range of tens or hundreds ofdegrees Celsius, because of the strain sensitivity of the FBG. Ifencapsulated in a material such as an epoxy or another material that isnot “perfectly elastic” (i.e., a material that is subject to measurableviscous flow), the mechanical stiffness of the fiber causes the fiber to‘creep’ or move through the viscous material when stressed by changes intemperature or mechanical causes. This occurs even if the length of theattachment or encapsulation greatly exceeds the length of the FBGitself. In addition, the viscous material itself is often not stableunder thermal cycling, especially if it is a glass with a low meltingpoint or is a polymer and its glass transition temperature is exceeded.These effects can lead to variations of temperature calibration of manydegrees Celsius from cycle to cycle and even to the loss of opticalsignal through the gradient-induced breakup of the single reflectionpeak into multiple peaks (termed accidental chirping, in contrast to theintentional variation of the period of a grating during fabrication).

While it can be very difficult to measure strain without temperatureeffects, measuring temperature without strain affects can be done withvarying degrees of success with appropriate packaging in order to removethe FBG from the effects of stress due to handling or attachment toanother object. Although such packaging inevitably increases thedimensions, mass and thermal response times of the FBG sensors, suchpackaging is necessary to make the sensors of general use in industry.On the other hand, it is extremely desirable to make fiber optictemperature sensor packaging as small and thermally fast as possible,and further to emulate the form factors of commonly used electronictemperature sensors to promote the market acceptance of the neweroptical technology in the marketplace.

In order to make the sensors in a physically single ended, ‘probe’configuration such as is easily done with thermocouples and thermistors,with both fibers coming out of the same end of a small tube or otherpackage, the fiber may be bent in at least a 180° ‘hairpin’ curve in away that avoids losing significant light transmission (a few tenths of apercent per sensor may be permissible in a sensor array of 100 sensors,for example). Conventional communications-grade optical fibers (e.g.,Corning SMF-28) begin losing significant amounts of optical transmissionwhen bent in diameters as large as 30 mm.

FIG. 1 shows example experimental data on power loss from single 180°bends in three types of optical fibers. For a single sensor on a fiber,losses of 50-90% might be tolerated, but if several are to bemultiplexed on a fiber, losses of less than 1% are desirable. From thepoint of view that multiplexing can be highly beneficial for lowestsystems costs, fewest fibers and feedthrough points, etc., it is evidentthat low loss can be important. FIG. 1 demonstrates that commoncommunications fiber with a numerical aperture of much below certainnumerical apertures may present challenges for use to make a compactsensor, but if the numerical aperture is increased, smaller sensors canbe fabricated that are also low loss. Generally speaking, a startingfiber with any numerical aperture may be used when a section of thefiber is drawn and tapered to an air-guided core (e.g., fiber drawn downto only a few micrometers in diameter). In that case, the index contrastbetween the glass and the air (n=1) is essentially the index of theglass, or 1.4 to 1.5, so the numerical aperture of the air-guided corefulfills the requirement of very high NA no matter what the starting NAwas.

The data shown in FIG. 1 does not address concerns with the well-knownincreased static fatigue failure of optical fiber as the bend radiusdecreases below about 3 mm; it is an illustrative example only of theoptical power loss due to the bend. For example, the estimated lifetimeof a buffer-coated fiber (that has never been stripped and recoated)bent in a 3 mm radius is greater than 50 years while a bend of 1.5 mmradius would have a failure time measured in hours. To avoid any suchincrease in fatigue failure for bend radii less than about 2 to 4 mm,thermal bending and annealing of the fiber comprising the bend isdesirable. If the thermal process does not induce an adverse chemical orphysical change in the core of the fiber, the benefits of the highnumerical aperture fiber will be retained and the probability of fiberfracture will be significantly reduced.

In general use, a fiber optic sensor package with a width or diameter of20-30 mm or greater is highly undesirable. Since electronic industrialsensors frequently are packaged in tubes with diameters of 0.5 to 13 mm,optically double ended, physically single ended fiber optic temperaturesensor probes with diameters of 0.3 to a maximum of 13 mm, andpreferably 0.3 to 6 mm, will find enhanced utility in industry. Thisdiscussion of round or tubular sensor probes does not exclude othercross sectional geometries, such as rectangular or oval cross sections.

The exemplary illustrative technology herein provides compact, opticallydouble-ended sensor probes with at least one substantially 180° bendprovided in the optical fiber in close proximity to an FBG sensor. Thisexample non-limiting structure may include for example all versions ofat least net 180° bends in definition and bends of somewhat less than180° that would lead to slightly divergent input and output fibers butstill allow a physically single-ended probe configuration within adesired maximum diameter. Further, the FBG sensor can in examplenon-limiting implementations be suspended in the probe in such a waythat the expansion and contraction of the probe casing will notmaterially influence the temperature reading of the FBG by addingtime-or-temperature varying stress components to the FBG. Suchtime-dependent drift mechanisms that can be avoided include creep inreading (at a constant temperature) that frequently occurs when attemptsare made to fasten fibers incorporating FBGs at both ends of the FBG tothe casing in a direction substantially on a line with each other, evenif said fiber is bent somewhat (substantially less than 180°) to preventfiber breakage.

Mechanical 180° bends can be mechanically restrained to force them intoa compact form factor if means are employed to prevent such restraintsfrom themselves causing variations in the calibration of the sensorswith time and temperature cycling. Thermally formed bends can be made byheating the fiber beyond its softening point utilizing any of themethods of, but not exclusively confined to, a flame, an oven, a hotfilament, a glow bar, or a laser, for instance a CO₂ laser. The buffercoatings can be removed before heating, burned off during the bendingoperation or, if an inert atmosphere is employed, an adherent,protective carbon layer can be left on the fiber bend. Reliability ofthe bend can be enhanced by annealing and slow cooling the bend. SinceFBGs in many fibers can be erased by high temperature, the FBG can be ofa type that can withstand the temperature of the bending operation, itcan be written into the fiber before bending and kept a safe distanceaway from the bend or the fiber can be loaded with hydrogen (ifrequired) after the bending operation and the grating can be writteninto the bent fiber after the hydrogen loading step.

Additional exemplary illustrative non-limiting features and advantagesinclude:

-   -   A compact optical fiber temperature sensor that is optically        double ended and can be made either physically single or double        ended as a probe or in-line, respectively, encompassing at least        one FBG in close proximity to at least one bend in the fiber        comprising at least one net 180° path    -   said fiber having a numerical aperture of much above a certain        numerical aperture such as about 0.11 in one particular        non-limiting example    -   said FBG further mounted within an outer casing    -   the optical fiber is a single mode fiber    -   the radius of the largest at least one net bend is from 0.01 mm        to 10 mm and preferably from 0.15 mm to 5 mm    -   an arrangement of fiber and FBG is mounted and maintained in        physical independence of expansion and contraction of the outer        casing, including rubbing on the casing.    -   mechanical stress placed upon the outer casing, as in fastening        said casing to an object to be measured for temperature, has no        or substantially no effect on the temperature calibration of the        FBG    -   the input and output fibers emerge or can be caused to emerge        from the casing essentially at the same end and substantially        parallel to each other in a probe configuration    -   the application of a bend or bends of greater than 180° or        multiple bends can cause the input and output fibers to emerge        from the casing at up to 180° from each other (i.e., at 90° or        180° substantially orthogonal or parallel to each other)    -   said casing contains an atmosphere    -   said casing is environmentally and/or hermetically sealed    -   the at least one FBG is contained in a straight section of fiber        within the distance of from 0.01 mm to 100 mm from one end of        the at least one 180° bend in the fiber and preferably from 1 mm        to 10 mm    -   the at least one FBG is at least partially contained within the        at least one net 180° bend in the fiber    -   the FBG is fabricated in un-stripped fiber according to the        definition    -   the at least one 180° bend in the fiber is formed by at least        one method chosen from the group mechanical bend, thermal bend        or tapered bend (per the definitions)    -   the at least one bend contains at least one length of        longitudinally tapered index of refraction, exclusive of the FBG    -   the at least one net 180° bend in the fiber is confined and        supported by at least one rigid band across approximately the        diameter of the bend and weighing between 10 ng and 10 g, and        preferably between 10 μg and 100 mg, such that the weight of the        at least one band is supported entirely by the fiber, free of        contact with the casing, and further the at least one band moves        freely with the fiber within the sensor casing without adding        variable stress to the FBG    -   the casing is made entirely of dielectric materials    -   the casing is hermetically sealed    -   the hermetic seal is chosen from one of the group of a weld, a        metal alloy solder or sealing glass composition    -   the optical fiber in the area of the hermetic seals is provided        with at least one of or a combination of a solderable metal        coating, an organic buffer coating or no buffer coating    -   the metal alloy solder contains at least one rare earth element.    -   the FBG is contained within a 360° bend    -   the 360° bend is substantially circular and the diameter of said        circle of fiber is fixed by at least one support at the point of        closure of said circle of fiber and which further forces the        input and output fibers to emerge substantially tangentially to        and near the plane of said circle    -   the length of the FBG is bare (without a buffer coating)    -   the FBG is protected from damage by at least one support with at        least one projection running substantially parallel to the FBG        and weighing between 10 ng and 10 g, and preferably between 1 μg        and 10 mg    -   said longitudinal support is a tube encompassing the FBG and        unattached to the fiber (i.e., floating freely on the fiber)    -   said tube is attached to the fiber or other portion of the        support structure on only one end, and in which the fiber is        free to expand and contract independently of said tube    -   the tube is composed of one or more of a metal, metal alloy,        glass, ceramic, composite or polymer    -   the atmosphere in the casing contains helium gas for the purpose        of enhanced thermal conduction between the casing and the FBG    -   the optical fiber containing the at least one 180° bend is holey        fiber, nanostructured fiber or photonic crystal fiber    -   a bend in an optical fiber in which the optical intensity losses        are reduced by increasing the index of refraction of the fiber        core within only the bend by means of exposure to ultra violet        radiation at least over a portion of the length of the bend    -   a bend in an optical fiber in which optical fiber within the        bend is subjected to ultraviolet radiation in order to increase        its numerical aperture, in which process said deep ultra violet        radiation is any combination of constant and varying intensity        over the length of the bend

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better and morecompletely understood by referring to the following detailed descriptionof exemplary non-limiting illustrative embodiments in conjunction withthe drawings of which:

FIG. 1 shows example experimental data on power loss from single 180°bends in three types of optical fibers;

FIGS. 2A-2E show different views of one example illustrativenon-limiting example implementation providing a competitively small,physically single-ended and yet optically double-ended, strain-freetemperature sensor probe;

FIGS. 3A-3D show illustrative examples of one type of compact casing inwhich the fiber/FBG assembly of FIG. 2A may be encased and protected;

FIGS. 4A-4D show an illustrative example of a fiber bent by thermalmeans, not requiring a physical restraint means across the diameter ofthe bend;

FIGS. 5A-5C show an exemplary illustrative non-limiting implementationof a physically single-ended but optically double-ended bent fiber FBGtemperature sensor probe incorporating a cylindrical casing and adesired seal;

FIGS. 6A-6C show a further exemplary illustrative non-limitingimplementation of a physically single-ended but optically double-endedbent fiber FBG temperature sensor probe incorporating a casing with anoval cross-section;

FIGS. 7A-7D show an exemplary illustrative non-limiting implementationof a miniature FBG loop temperature sensor incorporating a 360° uniformbend;

FIG. 8 shows a further exemplary illustrative non-limiting example of a360° loop sensor incorporating fibers further bent an additional 90°;

FIGS. 9A and 9B show a further exemplary illustrative non-limitingimplementation of a 360° loop temperature sensor in a racetrackconfiguration;

FIG. 10 shows an exemplary illustrative non-limiting implementation of amethod of making a permanent, thermally formed bend; and

FIG. 11 shows an exemplary illustrative non-limiting implementation of amethod of making a mechanical bend and applying mechanical constraints.

Note: The drawings herein represent the fiber in two dimensions whileassuming a buffer coating is included on the fiber except as notedbelow.

DETAILED DESCRIPTION

FIGS. 2A-2E show different views of one example illustrativenon-limiting example implementation providing a competitively small,physically single ended and yet optically double ended, strain-freetemperature sensor probe. In FIG. 2A, the input/output fibers 30, 30′shown in one example non-limiting implementation have numerical apertureof 0.11 or greater and a 180° uniform mechanical or thermal bend withuniform radius 35 less than or equal to 10 mm. At least one front brace32 is provided with its centerline substantially along the diameter ofthe bend 35 and at least one back brace 33 fixed to the fiber at leastat points 34 with a fixative. The purpose of said braces 32, 33 tomaintain the at least one FBG 31 free of longitudinal and/or bendingstresses, while being of low enough mass to prevent the fiber frombending substantially in the direction normal to the plane of the 180°bend under gravity or other forces. While said braces 32 and 33 may beof different materials, it is preferable that they be of the samematerial in order that the expansion and contraction of the braces underthermal cycling will be transferred to the regions outside the bandedlength containing the FBG, i.e., to the bend 35 and to the length offiber between band 33 and the casing fiber feedthroughs (see FIGS.3A-3D), thus further isolating the FBG from effects of the fibermounting, encasement and the manner of mounting of the casing to theobject to be measured. This feature enables the example, illustrativenon-limiting FBG to have the buffer layer removed without causingvulnerability to static fatigue fracture or vibration, further enhancingits temperature repeatability by the removal of all dissimilar materialsfrom contact with the FBG. In this manner, the braces may be of a highsilica glass that closely matches the fiber thermal expansioncoefficient (TCE), a metal, or a polymer that are highly mismatched tothe TCE of the fiber. Also indicated are directions 37-39 for which thetemperature-induced motion of the FBG is independent of the TCE of itscasing and the influence of any object to which the casing is attached,excepting the temperature of said object. The fixative 34 may be forexample but non-exclusively an epoxy or other glue (loaded with aninorganic material or completely organic), a melted polymer orfluoropolymer, a ceramic material, or a silica-based sealing glassmaterial

If the fiber buffer coating is removed in most of the area between thebraces 32, 33, the only material or component affecting the temperaturesensitivity and temperature reproducibility of the FBG is the glassfiber itself. Thus in the direction 38, the FBG is not affected even bydifferential expansion coefficients of a buffer coating andirreproducibilities due to shear forces between the buffer coating andthe glass, which can cause slippage or yield and thus cause significantchanges in temperature calibration with time. On the other hand, in thedirections typified by 37, the braces 32, 33 expand and contractidentically, keeping the two legs of the fiber between them parallel andtransferring stress to the non-sensing portions of the structure, namelythe bend of radius 35 and the fiber lengths between brace 33 and thefiber feedthrough points in the casings 54, such as shown in FIGS.3A-3D. Thus braces 32, 33 can be made of a material with a muchdifferent expansion coefficient than that of the optical fiber, such asfor example polyimide.

FIG. 2B is an orthogonal view of the assembly shown in FIG. 2A with theaddition of the indicated direction 39 in which the fiber is free tomove without contact with the casing.

FIG. 2C is one illustrative configuration of braces 32 and 33, whichcontain the fiber by means of machined holes and a fixative.

FIG. 2D shows several illustrative configurations of braces 32 and 33,in which the fiber may be contain by means of notches or multipiececonfigurations. The fixatives are not shown for clarity.

FIG. 2E is the same as FIG. 2A with the addition of further protectionof the FBG in the form of a small, light weight tube 36, asnon-exclusive examples a metal hypodermic needle tube or a glasscapillary tube, which may or may not be round in cross section, whichmay or may not be of one piece and may or may not be radiallysymmetrical. Said protection tube may be free to move independently ofthe fiber expansion and contraction in the direction 38 or may beattached only at one end to the fiber or a brace without influencing thestability or temperature calibration of the FBG. Said protection tubecan be suspended free of contact with the area of the fiber containingthe FBG, wherein said FBG may or may not have its buffer coatingremoved, by ensuring that its ends ride on the buffer coating outsidethe length containing the FBG. In one exemplary illustrativenon-limiting implementation, the protection tube 36 may be quartz withboth ends fixed to the fiber outside the grating area. Also shown is anend view of the assembly indicating the small radius, light weightnature of the protection tube 36.

FIGS. 3A-3D show illustrative examples of one type of compact casing inwhich the fiber/FBG assembly of FIG. 2A may be encased and protected.The dimensions of said casing may be, as a non-exclusive example, 1 cm×2cm×0.4 cm or smaller. The casing 50 may be nonexclusively metal with acavity 51 to contain and protect the FBG in the bent fiber assemblywhile allowing said bent fiber assembly to move in any of directions 47,48 and 49 under thermal stimulus from the environment to be measuredwithout said casing or environment influencing the temperaturecalibration of the sensor or the reproducibility of the sensorcalibration. Said casing 50 may have a metal lid 52 that can besoldered, brazed or welded with lid seal 53, may be ceramic with aglass-frit-sealed lid or polymer with an adhesive sealed or welded lid.Any number of other casing shapes, types and configurations may beequally possible for different applications. Hermetic seals 54 betweenthe fiber 40 and the casing 50 combine to contain an atmosphere that maynon-exclusively contain a partial vacuum, air, an inert atmosphere or ahigh thermal conductivity gas such as helium. Preferably, saidatmosphere will contain at least a partial pressure of helium gas toenhance heat transfer between the suspended fiber assembly and thecasing. Said atmosphere will preferably have a boiling point below thelowest intended temperature of operation of the temperature sensor, andcould variously contain helium, hydrogen, neon, nitrogen, oxygen, argon,or a hydrocarbon, but should preferably exclude water vapor to slow anyfiber static fatigue damage. Said partial pressure of helium gas will bea preferred, high thermal conductivity addition to any atmosphere toprovide the widest operating temperature range with the fastest thermalspeed.

FIGS. 4A-4D show an illustrative example of a bent fiber, FBGtemperature sensor utilizing a thermally bent 65, annealed fiber thatmay or may not require or use the front brace 32, 42 of the mechanicallybent fiber assembly shown in FIGS. 2A and 3A respectively, but mayutilize at least one brace 62, 63 elsewhere. In this case, the preferredmaterial for the brace or braces will be as low an expansion coefficientmaterial as practical, approaching that of fused quartz, SiO₂.

FIGS. 5A-5C show a non-exclusive, illustrative example of a physicallysingle ended but optically double-ended bent fiber FBG temperaturesensor probe incorporating a cylindrical casing 90 with a cavity 91having an inside radius 95 of less than about 8.5 mm and furtherincorporating a rolled and welded distal seal 86 and a plug 92 thatcontains fiber feedthroughs 94 and seal 93, all of which may behermetic. The directions of free motion of the FBG assembly for whichsaid FBG is free of the influence of the casing and outside environmentexcept for temperature are shown as 87, 88, and 89.

FIGS. 6A-6C show a further non-exclusive, illustrative example of aphysically single-ended but optically double-ended bent fiber FBGtemperature sensor probe incorporating a casing 110/111 with an ovalcross section, which may alternatively be square or rectangular. Saidprobe casing additionally incorporates a sealed-on distal end 118 whichmay have a bolt hole 119 for attaching said casing to the object to bemeasured, or alternately may provide a convenient tab for welding thecasing to the object to be measured and is sealed with seal 113. Saidcasing also additionally illustrates robust terminations 115 attachingcable jackets 116 to the casing by means of crimp connections 117. Saidcabling may also contain strength members such as Kevlar fibers and asmaller diameter liner tube though which the optical fibers run.

FIG. 7A-7D show an illustrative example of a miniature FBG looptemperature sensor 121 incorporated into a 360° uniform bend in anoptical fiber 120 with radius 125 in a casing cavity 131 through seals134. Said casing may illustratively have dimensions of 1 cm×1 cm×0.4 cmor smaller and thus will greatly increase the adaptability of FBGsensors for strain free spot temperature measurement. The thermalresponse of the FBG in this configuration is substantially independentof the influence of the casing 130 and the manner of fixing to theobject to be measured. Further, because a single fixing point or brace123 with only a small amount of fixative 124 is used over only a verysmall portion of the bend, the fiber loop is free to expand and contractwithout disruptive influence of the supporting structure. Outside straininfluences do not reach the FBG because its diameter is fixed andsuspended in the cavity. A slight bend relief 135 between the loop 125 aand the casing 130 inside the cavity 131 prevents the expansion andcontraction of the casing plus the object to which the casing isattached for temperature measurement purposes from breaking or strainingthe fiber inside the cavity but outside the loop. This arrangement makespossible a very compact, physically double-ended (in-line) but stilloptically double-ended configuration. The desired three degrees ofstrain-free motion 127, 128, 129 for the grating are maintained.

FIG. 8 shows a further illustrative example view of a 360° loop sensor145 incorporating FBG 141 with the fibers 140 further bent an additional90° each in order to provide a physically single ended but stilloptically double ended temperature sensor. Said additional 90° bends canbe mechanically or thermally formed. Isolation from mechanically inducedstrain in the direction 148 is provided by the fiber sections 142between the casing feedthrough points 146 and the fixing brace 143 withfixative 144. Optionally, one or more braces could be added in the fibersections 142. The freedom-of-motion directions 147,148 are indicated,but the third orthogonal direction is also free of all influences excepttemperature.

FIGS. 9A and 9B show a further illustrative example of a 360° looptemperature sensor in a race track configuration with the at least oneFBG sensor 151 incorporated in a straight section of fiber 150 betweentwo braces 152 and 153 in order to provide a physically double-ended andoptically-double ended temperature sensor. Said two 180° bends can bemechanically or thermally formed. The straight sections of fibercaptured between braces 152, 153 allow the use of FBGs with the buffercoating removed without danger of static fatigue or stress failures.Isolation from mechanically induced strain is further enhanced by slightbends of radius 155 in the fiber sections 156 between the fixing points154 and the casing feed through points 159. The critical freedom ofmotion direction 158 is indicated along with the secondary direction157. The orthogonal direction of freedom is implied as illustrated inFIG. 7C, feature 129.

FIG. 10 shows a non-exclusive, illustrative example of a method ofmaking a permanent, thermally formed bend. A self-heated mandrel 161,which may be non-exclusively a graphite, Kanthal™ or Nichrome™ bar orwire, is clamped into buss bars 162, 163 with setscrews 164 or someother means and is heated by current 165. A thermocouple could be weldedto the mandrel or an infrared thermometer could be used to monitor themandrel temperature. Fiber 160 with FBG 167 fabricated in the fiber andis positioned properly for the desired location of the bend. When themandrel is hot enough, in the neighborhood of 700-1000° C., the fiber ismoved to positions 160(a) and the mandrel is cooled. The fiber is thenremoved from the mandrel and the burned-off buffer coating is replacedwith the same or another material. Alternately, if the heating isperformed in an inert atmosphere, an adherent carbon coating ofpyrolyzed buffer coating may be left on the fiber, forming a protectivecoating. Any braces desired are then added or the fiber is inserted intothe casing feedthroughs and affixed to the casing. A 360° C. bendcontaining a high temperature-tolerant grating can be accomplished inthe same manner, or the grating can be written in the fiber afterbending.

FIG. 11 shows a non-exclusive, illustrative example of a method ofmaking a mechanical bend. Two posts 174 and 176 are rigidly supported inrelation to each other in a fixture and are used to form the fiber 170containing the FBG 171 and position the braces 172, 173 for theapplication of the fixative 177 at each point where the fiber is to beattached to the braces. The radius of post 174 is that of the desiredfiber bend 175. The distance 178 can be reduced by one half thethickness of the brace 172 to center the fiber bend support at thediameter of the bend, or alternately the post 174 need not be half-roundin order to make desired manipulations of the fiber configuration. Thebraces 172 and 173 can further be clipped to the posts 174, 176 andother positioning and supporting elements 177 can be added as necessaryto prevent the fiber and braces from moving and to make the fabricationprocess efficient and accurate. It is highly desirable in one examplenon-limiting implementation to prevent the assembly from twisting so thefiber no longer lies in a single plane. The fixture can for example beinserted into an oven to cure the fixative. The sensor is then removedfrom the fixture and inserted into its casing.

SOME DEFINITIONS

Index contrast: The difference between the higher index of refraction ofthe fiber core and the lower index of refraction of the fiber cladding.

Bend-sensitive fiber (high loss with reference to bending): Numericalaperture lower than or equal to 0.15, usually designed to be low loss inboth the 1300 nm and 1550 nm wavelength bands—common communicationsfiber (e.g., Corning SMF-28 or 28e™)

Bend-insensitive fiber (low loss with reference to bending): Numericalaperture of at least 0.11 in some non-limiting implementations (anynumerical aperture can be used if fiber is drawn down to only a fewmicrometers in diameter)

Holey fiber (sometimes called a photonic crystal or photonic bandgapfiber): High numerical aperture fiber in which the high index contrastis provided by an array (usually a geometrically regular array) of holesin the cladding around the core of the fiber, and running parallel tothe core throughout the length of the fiber. May have a hollow core.

Nanostructured fiber: Fiber with a ring of nanostructures around thecore that produces the effect of a high numerical aperture fiber butallows a larger mode field diameter than bend insensitive fiber and goodtransmission in a wider band of wavelengths (e.g., Corning PhotonicsClearCurve® optical fiber made with nanoStructures™ technology;approximately 1285-1625 nm). Much smaller radius bends are possible thanwith the same company's SMF-28e™ fiber, but it is still fully compatiblewith SMF-28e™.

Uniform bend: A bend in the fiber made by mechanical or thermal meanswithout changing the diameter of the fiber materially.

Mechanical bend: A bend made with mechanical force and maintained with amechanical constraint that is mechanically stiff but light weight andsmall enough to move with the fiber without causing dragging on the caseor distortion to the FBG signal.

Thermal bend: a bend in the fiber made by heating it thermally above itssoftening point to permanently form the bend in a stress-free conditionwithout materially affecting the fiber diameter, after which the bendcan be recoated with a buffer coating to protect it.

Tapered or drawn bend: A bend in the fiber made by thermally heating thefiber above its softening point, stretching it so its diameter taperssmoothly (adiabatically) to a minimum and smoothly returns to theoriginal diameter, afterward forming at least one 180° bend eithermechanically or by further thermal treatment. Minimum diameter of a fewmicrons can reduce optical intensity losses to a few percent or less bycausing the light to be guided in the remaining glass with air as the‘cladding’ (air-guided fiber).

180° bend: Includes bends in the fiber that are of constant radius, amix of different radii and straight sections, a piecewise linear,segmented circle, an angle or a circle segment that is more than 180° orsomewhat less than 180°.

UV flood: Subjecting the length of a fiber bend to a fluence of deepultraviolet radiation of sufficient intensity such that the index ofrefraction of the fiber core is increased above the original index ofrefraction of the fiber core and over the entire length of the bend,thus increasing the numerical aperture and reducing the loss of lightintensity of the signal light in the fiber core.

Un-stripped grating: An FBG that is written during the fiber drawingprocess before the buffer coating is applied or is written through abuffer coating without stripping and recoating the buffer.

Optically double ended sensor: Sensor with two optical fibers emergingfrom the casing, in any direction, with either fiber being useable asthe input or the output fiber and able to operate either in reflectionor transmission.

Optically single ended sensor: Sensor with only one fiber entering thecasing and able to operate only in reflection.

Physically double ended sensor: Sensor with the input and output fibersemerging from the casing at substantially opposite ends andsubstantially parallel.

Physically single ended sensor: Sensor in a probe configuration withboth fibers emerging from the sensing portion of the casingsubstantially in the same direction

Fixitive: A material or method of producing a hard, rigid attachment ofan optical fiber to another structure or material.

While the technology herein has been described in connection withexemplary illustrative non-limiting implementations, the invention isnot to be limited by the disclosure. The invention is intended to bedefined by the claims and to cover all corresponding and equivalentarrangements whether or not specifically disclosed herein.

We claim:
 1. A compact optical fiber temperature sensor comprising: anoptical fiber including a bend on the order of 180 degrees with a radiusof curvature not in excess of 10 mm; a fiber Bragg grating defined on orin a portion of said optical fiber in proximity to or within said bend;and a casing that at least partially encases said optical fiberincluding said fiber Bragg grating and said bend and suspends theoptical fiber therewithin to isolate the optical fiber and fiber Bragggrating from thermal-dependent expansion and contraction of the casing,at least one brace in the interior of the casing attached to said fiberat two points with the fiber Bragg grating therebetween, the at leastone brace having an expansion coefficient substantially the same as theexpansion coefficient of the optical fiber and having a mass such thatit does not cause the optical fiber to move substantially in anydirection due to the force of gravity, wherein said fiber including saidBragg grating and said bend is suspended from the casing against gravityin all directions at the points of penetration of the optical fiberthrough the casing that are the only two points of contact of saidoptical fiber with said casing.
 2. The sensor of claim 1 whereinmechanical stress placed upon the casing has substantially no effect ontemperature calibration of the fiber Bragg grating.
 3. The sensor ofclaim 1 wherein said radius of curvature of said bend is in the range offrom 0.10 mm to 5 mm.
 4. The sensor of claim 1 wherein said opticalfiber includes first and second portions that can emerge from the casingat any angle to each other by the continuation of the bend angle togreater than substantially 180 degrees.
 5. The sensor of claim 4 whereinsaid optical fiber includes first and second fiber portions that emergefrom the casing parallel from the same side or end, parallel at theopposite sides or ends, or at 90 degrees to each other.
 6. The sensor ofclaim 1 wherein said casing contains an atmosphere.
 7. The sensor ofclaim 1 wherein said casing includes an environmental seal.
 8. Thesensor of claim 7 wherein said environmental seal comprises an adhesive.9. The sensor of claim 8 wherein said adhesive comprises epoxy.
 10. Thesensor of claim 1 wherein said casing includes a hermetic seal.
 11. Thesensor of claim 10 wherein the hermetic seal is chosen from one of thegroup consisting of a weld, a metal alloy solder or sealing glasscomposition.
 12. The sensor of claim 10 wherein the optical fiber in anarea of the hermetic seal is provided with at least one of or acombination of a metal coating, a solderable metal coating, an organicbuffer coating or no coating.
 13. The sensor of claim 10 wherein thehermetic seal comprises at least one rare earth element metal alloysolder.
 14. The sensor of claim 1 wherein the fiber Bragg grating iscontained within a 360° bend of said fiber.
 15. The sensor of claim 14wherein the 360° bend is substantially circular and the diameter of saidcircle of the optical fiber is fixed by at least one brace at the pointof closure of said circle of the optical fiber and which further forcesthe two fiber portions outside the bend length to emerge substantiallytangentially to and near the plane of said circle.
 16. The sensor ofclaim 1 wherein at least the portion of the optical fiber defining thefiber Bragg grating has no coating or protection other than the casing.17. The sensor of claim 1 wherein the casing contains one or more ofhelium gas and neon gas that enhances thermal conduction between thecasing and the fiber Bragg grating.
 18. The sensor of claim 1 whereinthe optical fiber comprises holey fiber, nanostructured or photoniccrystal fiber.
 19. The sensor of claim 1 wherein the fiber Bragg gratingis disposed on a straight section of said optical fiber within apredetermined distance from said bend.
 20. The sensor of claim 19wherein one end of the fiber Bragg grating is contained in a straightsection of the optical fiber within the distance of from 0.01 mm to 100mm from one end of said bend.
 21. The sensor of claim 1 wherein thefiber Bragg grating is at least partially contained within said bend.22. The sensor of claim 21 wherein the fiber Bragg grating is fabricatedin un-stripped fiber.
 23. The sensor of claim 1 wherein the bend isformed by at least one method chosen from the group of thermal bendingand tapered bending.
 24. The sensor of claim 23 wherein the bend isfirst formed and then coated with additional protective materials. 25.The sensor of claim 23 wherein the bend is formed by bending the opticalfiber in an inert gas such that the bend is left coated with an adherentcoating of carbon, requiring no further recoating.
 26. The sensor ofclaim 1 wherein the bend contains at least one length of longitudinallytapered index of refraction, exclusive of the fiber Bragg grating. 27.The sensor of claim 1 wherein said at least one brace is acrossapproximately the diameter of the bend and weighing between 100 ng and10 g, said at least one brace confining and supporting the bend suchthat the weight of said at least one brace is supported entirely by theoptical fiber, free of contact with the casing, and said at least onebrace moves freely with the optical fiber within the casing withoutadding variable stress to the fiber Bragg grating.
 28. The sensor ofclaim 1 wherein the casing is made entirely of dielectric materials. 29.The sensor of claim 1 wherein the optical fiber has a predeterminednumerical aperture of at least 0.11.